Due to their high storage density, long data retention life, and relatively low cost, optical disks are becoming increasingly popular as a means to distribute information. Large format disks have been developed for storing full length motion pictures. The compact disk (CD), and more recent mini disk (MD) formats were developed and marketed for the distribution of musical recordings and have essentially replaced vinyl records. Recognizing the limitations of floppy disks for the distribution of large programs and data bases, the optical format was adapted for use as a high-capacity, read-only data storage medium (CD-ROM) in the personal computer field. Even more recent developments in the application of optical disk technology, such as the MultiMedia CD (MMCD) developed by Sony Corp., the Super Density (SD) system developed by Toshiba Corporation, and the new Digital Video Disk (DVD) standard provide ever increasing storage capacities.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, is encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data is read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the laser's field of illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal which may be decoded to recover the digital data stored on the optical disk. The recovered digital data, which may include error correcting codes and additional subcoded information, is further processed to recover the stored data which may then be converted to audio signals, or used as executable programs and data depending on the type of optical disk being read.
To be able to retrieve data from anywhere on a optical disk, the optical and imaging systems include a pickup assembly which may be positioned to read data from any disk track. Servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity.
Because the data is retrieved from the disk serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch, is fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 .mu.m, while the proposed MMCD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers have focused on increasing the rate at which the pits pass by the pickup assembly by increasing the rotational speed of the disk itself. Currently, drives with rotational speeds of 2x, 4x, and 6x normal speed are commercially available, and 8x designs are on the horizon. However higher disk rotational speeds place increasing demands on the optical and mechanical subsystems within the optical disk player making such players more difficult and expensive to design and manufacture.
Other previously known techniques for increasing average data transfer rates involve methods to intelligently anticipate future read requests by a host processor. It has been observed that data access by computers frequently exhibit "locality of reference," which means that a future data access will be local, in either space or time, to a previous data access. Thus a CD-ROM drive or controller can "read ahead" and buffer the data that the host processor is likely to request next. When the host processor next requests data from the optical disk drive, the drive first checks if the requested data has already been read and buffered. If the data has already been buffered, the drive simply sends the buffered data to the host, avoiding the delays associated with repositioning the pickup assembly and reading data from the optical disk itself. While such caching techniques may speed up average data transfer rates, the maximum data transfer rate is still limited by the rotational velocity of the optical disk within the optical disk reader.
It would therefore be desirable to provide an optical disk reading apparatus and methods that provide high speed retrieval of information from an optical disk while avoiding the limitations imposed on optical disk rotation speeds encountered by previously known devices.
It would also be desirable to provide an optical disk reading apparatus and methods that provide high speed retrieval of information from an optical disk that permits accurate tracking of the data contained on the optical disk.